Since organic materials which consist of natural macromolecule, synthetic macromolecule, fats and oils, lubricants, working oil, or so on are subject to oxidation and decrease in utility, various antioxidants have been devised and have been added to these organic materials. It is known that stabilizers such as hindered phenol compounds, thioalkanoate compounds, organic phosphorus compounds and aromatic amines have stabilizing effects when used singly or in combination. The above-mentioned stabilizing agents have their respective merits and are useful. However, there have been strong demands in recent years for the improvement in processing technology and for high quality molded products. In particular, phosphite compounds which belong to organic phosphorus compounds are widely used as useful antioxidants.
As concrete examples of phosphite compounds, there have been known the compounds which are described in the gazettes of Japanese Patent Application Examined Publication (Kokoku) No. 1641/1958 and Japanese Patent Application Unexamined Publication (Kokai) No. 4629/1984. The attempts to obtain stabilizing effects by the combined use of phosphite compounds and hindered phenol compounds have been disclosed in the gazette of Japanese Patent Application Examined Publication (Kokoku) No. 12373/1962, in the specification of U.S. Pat. No. 3,558,554, in the gazette of Japanese Patent Application Unexamined Publication (Kokai) No. 109050/1976 and in the gazette of Japanese Patent Application Examined Publication (Kokoku) No. 21822/1987. Further, there are disclosed phosphite antioxidants in U.S. Pat. Nos. 4,163,007, 4,182,704 and 4187212.
However, the above-mentioned phosphite compounds hitherto known include various problems such that sufficient stabilizing effect cannot be expected since the phosphite compounds are susceptible to hydrolysis and thermal decomposition, and inconveniences such as corrosion and foul-smelling due to the decomposed product are apt to occur. To solve these problems, there have been proposed phosphite compounds improved in resistance to hydrolysis and thermal decomposition. However, such compounds are not satisfactory but pose another problem in that advantageous properties of the hitherto-known phosphite compounds, such as protection of coloring at an initial stage during addition and kneading and that of thermal coloring in resin processing, are impaired. For example, tris[n-octadecyl-.beta.-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]phosphi te [Japanese Patent Application Unexamined Publication (Kokai) No. 4629/1984], tris(2,6-ditert-butylphenyl)phosphite, etc. pose problems mentioned above, and in addition, the bulky structure in the neighbourhood of the phosphite bond renders synthesis thereof very difficult, making themselves inadequate as an antioxidant to be supplied in a large amount.
The present inventors synthesized and evaluated tris[2-tert-butyl-6-methyl-4-(2-octadecyloxycarbonylethyl)phenyl]-phosphit e in order to overcome difficulties in the synthesis of the above-mentioned tris compounds to find that heat stability was improved, whereas no improvement was observed in color tone.
Meanwhile, U.S. Pat. No. 4,182,704 discloses bis(2,6-ditert-butyl-4-ethylcarbo-n-octadecyloxyphenyl ester)phosphite. However, the compound is not sufficient in its effect, and the bulky structure of tert-butyl in the neighbourhood of the phosphite bond renders mass production thereof unattainable.